Air in the form of bubbles entrained in solutions, such as solutions in a photographic processing system or kettle, can result in both sensitometric and physical defects in the processed film. Bubbles are generally introduced as the contents of the kettle are being stirred but may result from other fluid dynamic steps, such as mixing in new components and liquefying solid materials. Any form of high shear mixing generally introduces bubbles as a by-product of the mixing process.
Since bubbles are generally not desirable in the final blended mixture, a number of schemes have been developed to eliminate bubbles from the solution prior to its next use in the process. There exist a number of patents that describe means for eliminating bubbles in a liquid flow, such as the apparatus disclosed in U.S. Pat. Nos. 3,904,392, 4,070,167 and 4,205,966. These patents, however, do not address the problem of detecting bubbles in the solution. Attempts to solve the problem of detecting bubbles in presumably bubble free solutions, are described, for instance, in U.S. Pat. Nos. 3,974,683, 4,138,879 and 3,283,562. The devices disclosed in these patents are designed to measure low levels of bubbles (i.e., occasional transient bubbles) in solution delivery streams and are generally not suitable for the cases of high bubbles levels normally encountered in a photographic processing chamber or kettle. There are also methods for extracting samples and measuring entrained air off-line by a Compressibility method or a density method. These methods are good for measuring high levels of entrained air, however, such techniques are inherently inadequate to solve the aforementioned problem because they are off-line and subject to the air content changing during sample handling.